Fabrication of high performance based deep-blue OLED with benzodioxin-6-amine-styryl-triphenylamine and carbazole hosts as electroluminescent materials

The present study reports synthesis of phenathroimidazole derivatives structures following donor–acceptor relation for high performance deep-blue light emitting diodes. Herein, methyl substituted benzodioxin-6-amine phenanthroimidazoles Cz-SBDPI and TPA-SBDPI derivatives that provide the blue light were designed and synthesized. These Cz-SBDPI and TPA-SBDPI show higher glass transition (Tg) temperatures of 199 and 194 °C and demonstrate enhanced thermal properties. Apart from enhanced thermal stability these compounds also exhibit superior photophysical, electrochemical and electroluminescent properties. The non-doped carbazole based device display improved electroluminescent performances than those of TPA-based devices. The strong orbital-coupling due to decreased energy barrier between Cz-SBDPI transitions result in deep blue emission with CIE—0.15, 0.06. For non-doped Cz-SBDPI device; high L (brightness):12,984 cd/m2; ηc (current efficiency): 5.9 cd/A; ηp (power efficiency): 5.7 lm/W and ηex (external quantum efficiency): 6.2% was observed. The results show that the D–A emitters can serve as simple but also as an effective approach to devise cheap electroluminescent materials that has high efficiency and can serve as OLED devices.

at designing donor-acceptor derivatives: (1) Cz-SBDPI and (2) TPA-SBDPI from triphenylamine (TPA) and carbazole (Cz) that act as strong and weak donors, respectively.These materials comprises of moieties that are involved in hole (donor) and electron (acceptor) transport and is expected to show high quantum yields in films.The low value of singlet-triplet (∆ ST ) splitting suggests that the energy available is sufficient only to excite the triplet state (E T ) of blue phosphorescent dopant.Therefore, this study focuses on synthesizing these derivatives that show balanced injection/transport abilities and impart high thermal stabilities required for the fabrication of high performance electroluminescent materials used as OLED devices.

Methods and spectral instrumentation
For thermal analysis, both DSC and TGA studies were conducted in N 2 atmosphere on SEIKO instrument (Model No. SSC5200) where heating rate of 10 °C/min was employed.Absorption spectra for both solution and films were monitored via UV-Vis spectrophotometer (Perkin-Elmer Lambda 35).Time-resolved fluorescence decay was monitored with Horiba Fluorocube-01-NL (Source: Nano LED; Detector: TBX-PS) and decay analysis was carried out by employing time correlated single-photon counting (TCSPC) method with DAS6 software to determine the Fluorescence lifetime for these emissive materials.Moreover, the validity and evaluation of data fitting was confirmed from the reduced χ 2 values.The fluorescence spectrometer (Model-F7100) was used to obtain quantum yield (PLQY).

Computational details
The parameters used for geometrical optimization involve ground state (DFT)/excited states (TD-DFT) carried out using Gaussian-09 Software 20 .Computational studies were performed to theoretically calculate the Energy for Oxidation (E ox ) and also HOMO energy computed by the formula: E HOMO = − (E ox + 4.8 eV).Also, LUMO energy was calculated by deducting the band gap from HOMO values by the following equation, shown below: E LUMO = E HOMO − 1239/λ onset .

Results and discussion
Here, the different materials used as blue emitters;

Thermal analysis
Thermal based properties of prepared electrolumFinescent materials was conducted using TG and DSC analysis.The fabricated materials exhibited superior thermal stabilities courtesy to the inclusion of bulky moiety at imidazole carbon that renders it highly rigid.Moreover, the capping effect noticed at side chains of imidazole results in increase in the size of nitrogen atom that provides enhanced thermal stability (T d and T g ) so that the devices can work efficiently (Table 1).Thus, increase in T g support amorphous nature and stable film formation via thermal evaporation, vital for OLEDs applications.It is also evident that Cz-SBDPI and TPA-SBDPI in particular demonstrate higher decomposition temperatures (T d ) of 568 and 554 °C, respectively.Similarly, glass transition temperatures (T g ) of 199 °C for Cz-SBDPI compared to 194 °C for TPA-SBDPI was also noticed (see Fig. 2).The enhancement in thermal stability associated with Cz-SBDPI in comparison with TPA is attributed  to the increased rigidity noticed in the case of Cz-SBDPI useful as OLED.Moreover, higher Tg values also indicate that the stronger intermolecular forces in bond formations for C-N⋯H as well as C-N⋯Ph interactions because of amine groups and moieties present in phenanthrimidazole plane results in close confinement and tighter packing leading to higher rigidity 21 .
The stability of Cz-SBDPI and TPA-SBDPI thin films at different temperatures, 40-90 °C for 12 h was examined by atomic force microscopic (AFM) technique.The RMS of 0.33 nm (Cz-SBDPI) and 0.36 nm was noted for TPA-SBDPI.The thin-film surface depicts that no significant morphological changes occur prior to and later on annealing at 90 °C for 10 h which indicate that prepared samples are stable with respect to the variation in temperatures and thus confirms that the fabricated emissive materials are appropriate for use as OLED devices (Fig. 3).

Photo-physical properties
Optical properties confined to electronic spectral studies of Cz-SBDPI and TPA-SBDPI properties were determined for both solution and solid state shown as Table 1 was done by recording absorption (λ abs ) as well as emission (λ emi ) spectra, respectively (see Fig. 4).The attachment of the nitrogen atom in phenanthrimidazole with that of the aryl group is characterized by the λ abs at 262 nm, whereas λ abs at 253 nm is ascribed to transition, π → π* arising from the styryl phenanthrimidazole ring and from intramolecular charge transfer that occurs between donor (carbazole/triphenylamine) to the acceptor (phenanthrimidazole), respectively 22 .Furthermore, the reason for the absorption in Cz-SBDPI and TPA-SBDPI was studied by computational methods.
Both, derivatives, Cz-SBDPI as well as TPA-SBDPI display blue emission observed at 410 and 414 nm, respectively.This phenomena is due to the absorption that results when charge transfer from donor to acceptor occurs intramolecular manner [21][22][23] .The developed Cz-SBDPI and TPA-SBDPI had excellent low-efficiency roll-of in a wide range of current density values.Furthermore, it was found that Cz-SBDPI and TPA-SBDPI in solution/ film exhibited PLQY values of 0.80 and 0.75, respectively.It was also noticed that the blue shift was increased with higher values of molar absorptivity.The existence of strong and weak electron donors: triphenylamine and   carbazole moieties is expected to enhance the efficiencies 24 .Since, core fragments in Cz-SBDPI and TPA-SBDPI are same, the newborn emitters exhibit identical absorption band.However, the EL spectra shown in Fig. 4 for Cz-SBDPI, the emission maximum was blue-shifted to 413 nm compared to TPA-SBDPI (416 nm), which induces an elongated conjugation that is stable over a wide range of applied voltages.Thus, high fluorescence efficiencies of Cz-SBDPI and TPA-SBDPI established could serve as an efficient and effective deep blue emitters.
In solution, small shift to longer wavelength with respect to their corresponding film was noticed.The stacking in the solid state (films) leads to suppression of π-π* transition and accounts for this observation 25 .Similarly, the red shift of the emission peak was more pronounced with the increase in the polarity of the solvent.This variation is attributed to the changes in the polarization-induced spectral shift 26 .Hence, Cz-SBDPI show higher blue shift in both absorption and emission in comparison to TPA-SBDPI, credited to lower capability of electron donation for Cz with respect to TPA.As expected, the most probable reason that can be accounted for the blue shift is due to the reduction or suppression in CT in S 1 emissive state with increase in LE composition.It is understood that enhancement of LE components favours red shift whereas CT reduction tends towards red shift of PL spectra.From the data obtained through experiments, it is very obvious that factor involved in CT suppression is more dominant compared to the one responsible for LE component enhancement.However, the increase of LE character results in overlapping of spectra (UV-Vis and PL) for TPA-SBDPI and Cz-SBDPI but not noticed in their parent compounds.The red shift as shown in Fig. 5 was higher in the case of TPA-SBDPI (64 nm) than Cz-SBDPI (35 nm).This observation is attributed to the solvatochromic effects that indicate the presence of CT character for excited state S 1 at lower energy levels of TPA-SBDPI and also in Cz-SBDPI [26][27][28] .Likewise, in absorption bands as depicted in Fig. 6, smaller deviations of 15 and 23 nm were noted for Cz-SBDPI and TPA-SBDPI, respectively.The device's light distribution pattern was examined, and the results showed that the Lambertian factor was between 0.80 and 0.75 (Figs.S1,S2, Supporting Information).Table 1 provides a summary of all the device performances.

HOMO and LUMO
The optimization of geometries for Cz-SBDPI and TPA-SBDPI was carried out with basis set, DFT/B3LYP/6-31G (d p) of Guassian 09 software.The optimized geometries in conjunction with their respective molecular orbital distributions, i.e., HOMO and LUMO is illustrated in Fig. 7.It was found that most of the phenanthrimidazole fragment and styryl moiety were located in HOMO orbital while Cz and TPA fragments were distributed in the LUMO orbitals.The presence of holes and adequate separation of electrons between HOMO and LUMO orbitals is responsible for the electron transport function observed in Cz-SBDPI and TPA-SBDPI.The direct value of HOMO energies of TPA-SBDPI (− 5.40 eV) and Cz-SBDPI (− 5.45 eV) were obtained from the oxidation onset potential of TPA-SBDPI (0.62 V) and Cz-SBDPI (0.58 V), respectively.This also confirm that it can act as a bipolar material.Thus, the hole and electron transport for these derivatives have to be good which is evident from its property of oxidation-reduction action.To address this issue, during calculation, simultaneous relaxation for entire geometrical parameters were done whereas the angles of torsion associated with these parameters were changed in a systematic manner from 0° to 360°.The surface energy diagram shown in Fig. 8 reveals that the minimum conformation energy corresponding to that in which the nitrogen atom (N23) of the imidazole is slanted at the angle, 92.72° (Cz-SBDPI)/110.2°(TPA-SBDPI).The attachment with the methyl substituted  benzodioxin-6-amine ring and imidazole carbon atom (C25) occurs at 98.5° Cz-SBDPI)/108.4° (TPA-SBDPI) because of two types of phenyl rings present in cores of carbazole and triphenylamine, respectively.In view of the non-coplanar geometry and rigid molecular back bone, the orthogonal dihedral angles of these compounds (Cz-SBDPI and TPA-SBDPI) indicates the conformational twists for Cz-SBDPI and TPA-SBDPI [29][30][31] and it is by virtue of this non-planarity of the conformation that suppresses the red shift by restricting intermolecular interaction and thereby facilitates the harvesting of high quantum efficiency (η ex ) in films.Moreover, the presence of large moiety at C(25) in addition to side capping at N( 23), provide high rigidity, increases the size which is responsible for blue emission and also enhances the thermal stabilities (T d and T g ) fulfilling all the criteria needed for efficient OLED devices (See Table 1).Also, the OLEDs lifetime are improved by high T d and T g values by forming thin films upon vacuum evaporation.
The wide band gap leads to increased triplet exciton lifetime which results in reduced lifetime of blue OLEDs.Developing deep blue emitters that has high efficiency and lifetime is crucial 32 .In recent past, improvements in both efficiency and lifetime for blue OLEDs has been the focus and research work is been carried out in this regard 33 .Consequently, emphasis on developing blue OLEDs that are efficient and have long-lifetime from phosphorescent and thermally activated delayed fluorescence (TADF) emitters is top priority for OLED device manufacturing industries.The device's lifetime of blue OLEDs are Cz-SBDPI 0.15, 0.06 and TPA-SBDSP 0.15, 00.7.To the very best of our knowledge, the device efficiency data of Cz-SBDPI and TPA-SBDPI and current state-of-the-art literature data for blue-emitting OLEDs reported recently have been compiled and presented as Table 3 [34][35][36][37][38][39][40][41][42][43][44][45][46] .
It is worthwhile to mention that the modifications in the thickness of the emissive layers to improve the efficiency and also attempts to increase the radiative rate will be the subject of investigation for our future studies.For weak donor carbazole substituted with phenanthrimidazole, the current and power efficiencies observed are 0.88 cd/A and 0.30 lm/W, respectively.The substituted carbazoles used in study performed by Gao et al. 54 had values of 0.65 cd/A and 0.48 lm/W as current and power efficiencies.However, in our case, the efficiencies obtained for Cz-SBDPI were 5.9 cd/A and 5.7 lm/W, much higher than previously reported studies.Therefore, it can be concluded that both Cz-SBDPI and TPA-SBDPI are most appropriate as fluorescent based OLEDs materials.properties and provides a new route where bis phenanthrimidazoles with donor-acceptor molecular structures in non-doped devices and bipolar electroluminescent materials produce high performance OLED devices.Thus, the derivatives shows balanced injection/transport ability in addition to high stability that results in outstanding device performances.Hence, these carbazole donors are beneficial as they specifically generate short-wavelength based absorption or emission and are useful as blue-light-emitting materials for optoelectronic applications.The findings of the present investigation illustrate a new route to fabricate luminescent materials with donor-acceptor molecular structure that can be used to harvest efficient device performances.

Figure 5 .
Figure 5.The red shift effect for (a) Cz-SBDPI and (b) TPA-SBDPI phenanthrimidazoles in different solvents monitored via UV-Vis spectra.

Figure 6 .
Figure 6.The red shift effect for (a) Cz-SBDPI and (b) TPA-SBDPI phenanthrimidazoles in different solvents observed from PL measurements.

Figure 7 .
Figure 7.The optimized geometries along with their HOMO and LUMO for Cz-SBDPI and TPA-SBDPI phenanthrimidazoles.

Figure 9 .
Figure 9. Representation of energy levels for non-doped devices of Cz-SBDPI and TPA-SBDPI phenanthrimidazoles.

Figure 11 .
Figure 11.Evaluation of plots: (a) luminance vs. voltage; (b) external quantum efficiency vs. current density; (c) current efficiency versus current density and (d) power efficiency versus current density to determine the performance of Cz-SBDPI and TPA-SBDPI phenanthrimidazoles as electroluminescent materials.

Table 2 .
Parameters associated with performance evaluation as EL device for Cz -SBDPI and TPA-SBDPI, respectively.

Table 3 .
Summary of recent developments and state-of-art on efficiencies of blue-emitting OLEDs devices.